Which of the following leukemias can be excluded as a possible diagnosis if Auer rods are seen in blasts on the peripheral blood smear?

The presence of Auer rods in blasts on a peripheral blood smear is a significant morphological feature that helps in diagnosing specific types of acute leukemia. Auer rods are cytoplasmic inclusions that are found primarily in myeloid cells and are indicative of acute myeloid leukemia (AML) or myeloid differentiation.

Acute lymphoblastic leukemia (ALL) is a type of leukemia that typically arises from lymphoid stem cells and is characterized by the proliferation of immature lymphoblasts. Unlike myeloid blasts, lymphoid blasts in ALL do not form Auer rods. Therefore, if Auer rods are observed in the blasts, it strongly suggests a diagnosis involving myeloid lineage, allowing for the exclusion of ALL as a potential diagnosis.

When considering acute myelomonocytic leukemia, acute myeloid leukemia without maturation, and acute promyelocytic leukemia, Auer rods may be present as they are associated with myeloid lineage cells. This makes them viable considerations in the differential diagnosis when Auer rods are noted. Thus, identifying Auer rods on a peripheral smear provides clear evidence against acute lymphoblastic leukemia.